How to make style sidebar like this ?

Hi there,

can i see your website where you want those styles applied ?

this is my site : http://fajrialhadi.com/

If you can add a Title to the Posts widget then i can look at the CSS the Titles require.

For the number count try adding this CSS:

.widget .wp-block-latest-posts li {
    position: relative;
    counter-increment: post-count;
    padding-left: 10px;
}
.widget .wp-block-latest-posts li:before {
    content: '#' counter(post-count);
    position: absolute;
    left: -30px;
    top: 5px;
    font-size: 25px;
    font-weight: bold;
    color: #016aff;
}